There always seems to be an assumption that it's the child who knows the truth who is going around telling all of the other kids just to be mean, but I bet in many of those situations the child was keeping quiet about not believing and wasn't actually trying to ruin anything, they just got asked questions and answered them honestly!
This is what I was getting at upthread with saying the kids aren't able to 'handle the truth'. They just say what they know to be true without thinking about the effect on others, or even if they know they should be hiding reality, they don't have the sophisticated thinking skills to dance around the questions, quickly come up with a 'non-answer', etc.
Actually figuring this sort of thing out in 'real time' is a learning experience for kids. When I was young, I started to 'figure out' that Santa might not be real when I was 6 or 7, about the same age as most kids, and start to ask my parents questions. When I asked "How can Santa get around the whole world in one night?" they would say things like, "Hmmm, I just don't know! How do you think he does it?" and other little 'clues' for me to go and figure it out a bit more.
Over time, I would see a twinkle in their eye when they answered me. They knew I had figured it out, but they were 'playing along' because I had a younger brother who still whole-heartedly believed (it's no coincidence that Santa always has a twinkle in his eye, because he is also in on the joke).
So I learned to 'play along' until my brother figured it out as well. And eventually he figured it out too, but because we both thought the presents would stop if we stopped believing, we all 'played along', despite the fact that now we all knew we were all in on the joke!
That's the bonding and the fun of Christmas. It's not a devastating revelation when you figure out the truth, because figuring it out is all part of the fun. This, I think, is what the OP doesn't get, because that's not part of the tradition where she comes from.